The present invention generally relates to the art of optical information processing and more particularly to an optical semiconductor device as well as to an optical processing system that uses such an optical semiconductor device.
With the persisting demand for high speed semiconductor devices, intensive efforts have been made for developing high speed compound semiconductor devices and integrated circuits thereof that have an operational speed exceeding the operational speed of conventional Si devices. A hot electron transistor (HET) is a representative example of such a high speed compound semiconductor device. Particularly, it is noted that there is a proposal for a multiple emitter HET that includes a plurality of emitters.
Further, the inventor of the present invention has proposed a multiple emitter heterobipolar transistor (HBT) having a structure somewhat similar to that of the multiple emitter HET in the U.S. Pat. No. 5,561,306 (patent application Ser. No. 08/295,538). In the proposed device, the base electrode is eliminated, and two different emitter regions are defined on a common emitter layer for carrying first and second emitter electrodes respectively. Thereby, one of the emitter electrodes is used for removing holes accumulated in the base layer. In the proposed device, the impurity concentration level of the n-type emitter is set to about 2.times.10.sup.18 cm.sup.31 3, which is substantially larger than the effective density of state of electrons (.apprxeq.5.times.10.sup.17 cm.sup.-3) in the emitter. Simultaneously, the impurity concentration level of the p-type base is set to about 1.times.10.sup.19 cm.sup.-3, which is substantially larger than the effective density of state of holes (.apprxeq.1.times.10.sup.19 cm.sup.-3) in the base. By setting the impurity concentration level of the emitter and base as such, it is possible to reduce the base resistance of the device significantly by removing the holes from the base by tunneling through the depletion region formed at the base-emitter junction, and one obtains a high speed semiconductor device. It should be noted that such a HBT has a distinct advantage over conventional HETs (that operate only in a very low temperature environment); they are operational also at a room temperature environment.
Meanwhile, there is a rapid development in the field of super high speed optical telecommunication networks, particularly in relation to the so-called multimedia applications. In order to realize a high speed, large capacity telecommunication network, it is not only necessary to use optical fibers in place of electric cables but also necessary to increase the transmission rate of the optical fibers from the conventional rate of 1 Gbit/s to at least 4 Gbit/s, and preferably larger than 10 Gbit/s.
When the transmission rate of the optical signals through the optical fiber network is increased as such, there naturally arises a demand for a photoreception device that responds to such a very high speed optical signals. Further, there exists various demands for other associated circuits such as a high speed preamplifier for amplifying the high frequency electric signals obtained by the photoreception device or a demultiplexer for extracting signals of desired channels from a multiplexed optical signal by conducting a demultiplexing process and for transmitting the same with a lower transmission rate.
FIG. 1 shows the construction of a conventional optical receiver including a demultiplexer.
Referring to FIG. 1, the optical receiver includes a photoreception device 103 such as a PIN diode that receives an optical signal from an optical transmitter 101 via an optical fiber 102, wherein the photoreception device 103 produces an electric output signal and supplies the same to a preamplifier 104 typically formed of a high speed transistor for amplification. The electric signal thus amplified in turn is supplied to a demultiplexer 105 for demultiplexing into electric signals of respective channels.
Generally, such an optical receiver has a drawback in that it requires a complex construction. Further, there is a drawback in that, while the PIN photodiode used for the photoreception device may have a sufficiently high response speed, the overall response of the optical receiver may be limited by the response of the preamplifier 104 or the demultiplexer 105. Thus, there has been a demand for a high speed optical receiver that has a simple construction and provides a high response speed suitable for processing high speed optical signals.